1. Field of the Invention
This invention relates to separation of isotopes by solar photoionization and particularly to separation of .sup.6 Li and .sup.7 Li.
2. Description of the Prior Art
There are two naturally occurring isotopes of lithium--.sup.6 Li and .sup.7 Li. The natural abundance of the isotopes is 7% .sup.6 Li and 93% .sup.7 Li. Both isotopes find several uses in purified form. .sup.6 Li undergoes fission on exposure to thermal neutrons, producing tritium. Thus, it has applications in nuclear weapons and, potentially, in fusion reactors. .sup.7 Li, in the form of LiOH or Li.sub.2 CO.sub.3, is used as a pH controller in nuclear reactors but has a much larger potential market as a heat exchanger fluid in nuclear reactors. High purity is necessary, because .sup.7 Li does not readily undergo fission upon exposure to thermal neutrons.
Laser methods for isotope separation are well known and have been described in both patents and the scientific literature. In recent years, several reviews of laser separation of isotopes have appeared (See e.g. Sov. J. Quant. Electron. 6, 129 (1976); 6, 259 (1976) and Scientific American 236, 2, 86 (1977)). In particular, laser-induced fractionation and separation of lithium isotopes have been described by Rothe et al. Chem. Phys. Lett. 53, 74 (1978); 56, 336 (1978). Their process involves sequential two-photon ionization of Li.sub.2. Initial excitation and ionization are both produced by laser irradiation (from one or two argon ion lasers). U.S. Pat. No. 4,149,077, issued Apr. 10, 1979 to Yamashita et al., discloses substantially the same method for laser separation of lithium isotopes. If large-scale lithium isotope production were based on this process, a great deal of expensive electrical energy would be consumed.
There are several other known processes for separating lithium isotopes--diffusion, mass spectrometry and electrolysis with an amalgam. These methods are also energy intensive and the amalgam process has pollution problems as well. Thus, a lithium isotope separation process which requires less energy and/or uses a renewable energy source would be attractive, particularly if it posed minimal pollution problems.